[qsort]

I don't doubt that what you say is true, but it strikes me as odd that they didn't sidestep the problem by picking an example where the variables are obviously uncorrelated.

It comes off as extremely sloppy: nobody expects kids in primary school to even understand what it means for variables to be independent, but certainly those writing the questions should know better!

[OJFord]

Just because they don't know better, or it's a mistake. I'm not suggesting they knowingly thought 'but that doesn't matter here we want a specific technically wrong answer'.